How do you have them set up to receive the email for both domains? Is it by adding the second (D2) email address to their user profile in AD users/computers? If so, this lumps all of their received mail together into the one mailbox -- is this the desired behaviour, except that they may need to send email as user@D2?
I've not been successful configuring one user with two exchange identities that the client can select from on client side in one outlook profile. My workaround would be in the user's Outlook, to create an internet (SMTP/POP3) email account, as the user@D2 and point it to your exchange server for the outgoing (SMTP) mail server -- assuming that the exchange server will allow the user to SMTP relay (either using SMTP authentication or allowing by internal IP only so you don't become an open relay). Disable the 'check this account during send/receive' or else they'll get duplicate emails in their mailbox (in fact, you may want to give it a bogus incoming mail server so the user can't mess this up by accident).
Having the internet mail account should enable the functionality for the user to select which account to send composed emails through, D1 is through their normal exchange mailbox and as D2 through the SMTP account. This will not work through OWA (Outlook Web Access).
There's another method too in which users' D1 and D2 mail would go into entirely separate mailboxes and the user would have to select, by Outlook profile which mailbox to connect to.
I know it's not step-by-step as you've requested but if necessary we can follow up with the step-by-step and/or more info via email.
--Charlie
